The research investigations discussed in this progress report include: (1) Studies on the kinetics of absorption, metabolism and excretion of potent skin photosensitizing agents like psoralen, 8-methoxypsoralen and trimethylpsoralen. (2) Studies on photoinhibition of the complement system in patients with erythropoietic protoporphyria, who are unusually sensitive to ultraviolet and visible radiation. (3) The pharmacologic and therapeutic studies concerning the use of oral beta-carotene for amelioration of photosensitivity reactions in 53 patients with erythropoietic protoporphyria, 25 normal individuals and 10 patients with polymorphic photodermatoses. (4) Studies on the ultraviolet (290-320 nm) light-induced erythema as a predictive marker of skin cancer. (5) Evaluation of highly effective topical sunscreen formulations for prevention of sunburn reaction and for prophylactic approach against skin cancer and solar keratoses. (6) Results concerning the evaluation of systemic psoralens (8-methoxypsoralen and trimethylpsoralen) plus sunlight or high intensity ultraviolet radiation (320-400 nm) from a specially designed light therapy unit for repigmenting the vitiliginous skin of patients with vitiligo. (7) The effects of ultraviolet laser irradiation (lambda 337.1 nm) in human skin and guinea pig skin.